Method for alloying an electrode with a semiconductive body



June 7, 1967 E. F. JOHNSON 3,323,215

METHOD FOR ALLOYING AN ELECTRODE WITH A SEMICONDUCTIVE BODY Original Filed Nov. 28, 1962 s sheets-sheet 1 Nd m GD WETT/NG A NGLE -0E6REE5 88%886 200 500 4.00 500 600 TEMPERATURE 056E562) CENT/GAA a:

d g II -i': 4.0 g I CONSTHAINED E 3.0 k $1 .0 E 2 I (/NcaMsMA/NED 0 100 200 300 o 500 600 Q TEMPERATURE 0 JACQUES A. 57.!100/5 June 27, 1967 IE. F. JOHNSON 3,328,215

METHOD FOR ALLOYING AN ELECTRODE WITH A SEMICONDUCTIVE BODY Original Filed Nov. 28, 1962 3 Sheets-Sheet 52 3 9 36 55 {639 36 N l/EN TORS 51/?1. EJOl/MSO/V J c (IE5 R. SILOU/S June 27, 1967 F, JOHNSON 3,328,215

METHOD FOR ALLOYING AN ELECTRODE WITH A SEMICONDUCTIVE BODY Original Filed NOV. 28, 1962 3 Sheets-Sheet 3 if 6.5 57 j? k lT q .132 A 58 f I H //v vEA/TORS 6/ 59 [Iva F. Ja /v.50

J4 c uEs R. STLOU/S HGENT United States Patent 3,328,215 METHOD FOR ALLOYING AN ELECTRODE WITH A SEMICONDUCTIVE BODY Earl F. Johnson and Jacques R. St. Louis, Ottawa, 0n-

tario, Canada, assignors to Northern Electric Company Limited, Montreal, Quebec, Canada Continuation of application Ser. No. 249,640, Nov. 28, 1962. This application June 25, 1965, Ser. No. 467,012 5 Claims. (Cl. 148-179) ABSTRACT OF THE DISCLOSURE This invention describes a method for alloying an electrode with a semiconductive body. The method comprises the steps of initially supporting and locating a pellet of electrode-forming material at one surface of a semiconductive body in a manner such that the pellet loses physical contact with the support therefor and begins alloying with the surface of the semiconductive body at a temperature between the melting point of the pellet and a predetermined maximum alloying temperature. The temperature of the pellet and semiconductive body is then raised to the predetermined maximum alloying temperature in an atmosphere conducive to the alloying action.

This is a continuation of application Ser. No. 249,640, filed Nov. 28, 1962, now abandoned.

This invention relates'to a method for alloying an electrode with a semiconductive body and a jig useful in carrying out the method, and is particularly suitable to the manufacture of alloyed-junction transistors.

semiconductive devices such as transistors and crystal diodes are often manufactured by alloying one or more pellets of electrode-forming material with one or more surfaces of a semiconductive body such as germanium or silicon of predetermined conductivity type. In a typical method of alloying an electrode with a semiconductive body, a small pellet of indium is first placed in contact with a small wafer of N-type germanium. The assembly is then introduced into an oven and subjected to the usual heat treatment for alloying in the presence of a reducing gas. As the temperature is raised above the melting point of the indium some of the germanium becomes soluble in the melted indium and a melt of indium plus germanium is formed. During subsequent cooling the solubility of the germanium decreases, causing recrystallization of germanium atoms out of the melt, which rebuilds the previously dissolved germanium crystal structure, producing a regrowth layer containing a certain concentration of indium atoms. Thus, a P-N junction is formed between the regrowth layer and the N-type germanium.

A P-N-P germanium transistor can be manufactured by alloying individual pellets of indium with opposite surfaces of a small wafer of N-type conductivity germanium.

For reasons of economy, it is desirable to alloy as many transistors as possible in one alloying process. For this purpose various types of jigs having a plurality of individual transistor alloying stations have been used to assemble the parts to be alloyed prior to the alloying process.

The present application will be directed to a method for manufacturing a P-N-P germanium transistor and a novel jig for carrying out the method, it being understood that the inventive features disclosed and claimed herein are also applicable to the manufacture of other types of semiconductive devices.

Prior to applicants invention, it was well known to alloy a pellet of indium with a surface of a germanium wafer by locating the pellet on the surface of the germanium by means of a jig, and then heating the assembly in an oven in the usual way. However, the jig was not only used as a means to initially locate the indium pellet on the surface of the germanium wafer. The jig was also used as a constraint on the indium pellet to define its area of contact with the germanium surface throughout the alloying process. Once this area was predetermined by the physical characteristics of the jig, the depth of penetration of the indium atoms into the regrowth layer was controlled by the alloying temperature alone.

Such a method had several disadvantages:

(1) Any constraints in the alloying area tended to entrap oxygen at the points of constraint, allowing less volume for the reducing gas to circulate, and thus discouraging uniform wetting of the germanium surface by the melted indium. This resulted in poorer quality transistors because with a given volume of indium, the indium did not penetrate the germanium where no wetting occurred and did penetrate to an uncontrollable depth where wetting did occur.

(2) Any contact between the indium pellet and the jig during alloying encouraged sticking of the indium to the jig. Such sticking sometimes caused the finished prod not to become damaged. If the pellet remained stuck to the jig after alloying was completed, the transistor element could easily be destroyed when the jig was unloaded.

(3) The depth of penetration of the indium pellet into the germanium was difficult to control because with a defined area of contact between the indium and the germanium surface, the depth of penetration was controlled by temperature alone. It can be readily seen that changes in the alloying temperature caused proportional changes in the depth of penetration.

By experiment, applicants have discovered that the wetting angle between an indium pellet and a surface of germanium in the absence of physical constraint falls continuously with increasing temperature. The wetting angle is defined as the interior angle formed between the surface of the germanium wafer and a tangent to the indium-air interface at the junction of the germanium surface after alloying is completed. Once the wetting angle for an indium pellet of given volume has been determined by choosing the temperature, the area of indium that will be in contact with the germanium surface and the depth of penetration are readily predictable. By this discovery, applicants have shown that an indium pellet can be alloyed with a surface of germanium with greater control over the depth of penetration without constraints, thereby overcoming the above disadvantages of the prior art.

According to applicants invention, a method for alloying an electrode with a semiconductive body is provided comprising the steps of initially locating a pellet of electrode-forming material at one surface of a semiconductive body by means of a jig such that the pellet will lose physical contact with the jig and begin to alloy with the surface of the semiconductive body at a temperature between the melting point of the pellet and a predetermined maximum alloying temperature. The jig with the assembled pellet and semiconductive body is then raised in temperature in an atmosphere conducive to alloying to the predetermined maximum alloying temperature.

By providing that the pellet will lose physical contact with the jig while alloying is taking place, applicants have eliminated the constraints and permitted natural physical forces to take effect during alloying. Better circulation of the reducing gas is permitted and the difliculties of entrapped oxygen virtually disappear. Thus, complete wetting is induced to allow better control over the depth of alloying and thus over the quality of .a finished transistor. Only a small area of contact exists between the jig and the indium pellet during-and only duringthe initial warm up period, which greatly reduces the risks of sticking. In the absence of constraints, the determination of the depth of penetration of the indium into the germanium is shared between the temperature and the area of contact between the indium and the germanium surface. This permits greater control over the depth of penetration.

Another problem that has existed in the past was that commercially available indium pellets possessed a thin oxide coating that tended to prevent the indium from properly wetting the germanium surface. In some prior art methods, attempts were made to overcome this difficulty by applying weights to the indium pellets for crushing the indium shell and breaking open the oxide coating thus permitting the indium to flow and make contact with the germanium surface when the indium reached its melting point. However, the provision of weights produced hydrostatic pressure in the melted indium which had to be compensated by further constraints with their inherent disadvantages. Attempts were also made to chemically reduce the oxide coating on the indium pellets by preheating in a reducing gas as a separate step before commencing alloying. This was unsuccessful because during cooling, oxygen present with the reducing gas reoxidized the surface of the indium pellets. The new oxide coating was sometimes more detrimental to good wetting than the original coating. Applicants, according to the present invention, by eliminating the constraints have permitted better circulation of the reducing gas during alloying. The oxide coating on the indium pellets can thus be reduced during alloying in the heating cycle. Of course, the oxide coating can reform again upon cooling but this is irrelevant because the areas of the indium pellet required to be in contact with the germanium surface have already made such contact during the heating cycle.

Applicants, according to the present invention, have also provided a jig that is specifically adapted tocarry out applicants novel method. The jig comprises means arranged to support a semiconductive body and locating means arranged to locate a pellet of electrode-forming material at one surface of the semiconductive body such that when the assembled jig is heated in an atmosphere conducive to alloying, the pellet will lose physical contact with the locating means and begin alloying with the surface of the semiconductive body at a temperature between the melting point of the pellet and a predetermined maximum alloying temperature.

According to another aspect of applicants invention, the elimination of physical constraints lends itself to the manufacture of high quality transistors in one heat cycle. In manufacturing transistors, it is very important that the pellets of electrode-forming material to be alloyed with opposite surfaces of a semiconductive body are in axial alignment with each other at their point of contact with the opposite surfaces of the semiconductive body. Prior art methods that used separate heat cycle to produce each junction of a transistor suffered from the disadvantage that axial alignment of the pellets of electrodeforming material could not always be ensured. Applicants have been able to overcome this disadvantage by initially locating the pellets of electrode-forming material in axial alignment with each other at opposite surfaces of the semiconductive body and then alloying in one heat cycle.

Therefore, according to another aspect of the present invention, a method is provided for alloying a pair of electrodes with a semiconductive body comprising the steps of initially locating a pair of pellets of electrodeforming material at opposite surfaces of a semiconductive body by means of a jig such that the pellets will lose physical contact with the jig and begin alloying with the opposite surfaces of the semiconductive body at a temp rature between the melting point of the pellets and a predetermined maximum alloying temperature. The jig is constructed so that the pellets are initially located substantially opposite each other. The jig with the assembled pellets and the semiconductive body are then raised in temperature in an atmosphere conducive to alloying to the predetermined maximum alloying temperature.

In applicants preferred embodiments of the invention, the pellets are initially located in contact with the opposite surfaces of the semiconductive body such that the axis of one pellet normal to one surface of the semiconductive body is substantially in alignment with the axis of the other pellet normal to the opposite surface of the semiconductive body. To permit natural physical forces to take effect during most of the alloying cycle, it is preferable that the indium pellets lose physical contact with the locating jig about at the melting point of the indium pellets.

Applicants prefer to use a jig made of suitably oxidecoated grade 310 stainless steel. Applicants have found that such a material produces good results since negligible contamination is possible to prevent good wetting, problems of sticking are reduced, mechanical tolerances can be maintained during repeated use and jigs with many transistor alloying stations can be constructed with very close tolerances. It is to be understood that other materials could be used such as, for example, suitably oxide-coated synthetic ruby. Whatever material is chosen, it should be capable of being (a) machined with the necessary precision; (b) suitably oxidized to prevent sticking and should have a uniform heat transfer characteristic.

Preferred embodiments of the invention will now be described, by way of example, where the same reference designations will be used for similar parts in the figures of the drawings, in which:

FIGURE 1 is a sectional view of one station of a jig used in a typical prior art method for manufacturing alloyed-junction transistors;

FIGURE 2 is a sectional view of an alloyed-junction formed between an indium pellet and one surface of a germanium wafer illustrating the wetting angle;

FIGURE 3 shows a curve illustrating the relationship between the wetting angle and temperature;

FIGURE 4 shows the relationship between the depth of penetration of an indium pellet into the regrowth layer for the constrained and unconstrained cases as a function of the alloying temperature;

FIGURE 5 is a sectional view of one station of a jig prior to alloying according to one embodiment of the invention;

FIGURE 6 is a sectional view similar to FIGURE 5 after alloying has been completed;

FIGURE 7 is a horizontal section taken along lines 7-7 of FIGURE 6;

FIGURE 8 is a perspective view of the bottom plate of one station of a jig according to another embodiment of the invention;

FIGURES 9 and 10- are sectional views taken along lines 9.9 and 1010 of FIGURE 8 with the top plate of the jig added and the elements to be alloyed assembled.

FIGURE 11 is a sectional view similar to FIGURE 9 after alloying is completed; and

FIGURE 12 is a horizontal section taken along lines 1212 of FIGURE 11.

Referring to the drawings, FIGURE 1 illustrates a typical prior art arrangement of an assembled jig used in the manufacture of alloyed-junction transistors that will serve as a useful comparison between the disadvantages of the prior art and the subsequent advantages of applicants invention.

A jig 20 comprises a bottom plate 21 and a top plate 22. The bottom plate 21 has a central cavity 23 and an aperture 24. The top plate 22 has a central aperture 25.

A small wafer of semiconductive material 26 is supported upon the bottom surface of the cavity 23 and the top plate is placed on the upper surface of the bottom plate 21. The aperture 25 is large enough to locate a pellet of electrode-forming material 27 on the upper surface of the wafer 26.

The assembly is then heated in an atmosphere conducive to alloying to some temperature above the melting point of the pellet 27 which melts to form a domeshape upon the upper surface of the wafer 25. The assembly is then cooled and the wafer 26 is turned over so that the dome-shaped pellet 27 locates in the aperture 24. A second pellet (not shown) is then inserted in the aperture 25 and located on the opposite surface of the wafer 26. The heating process is repeated and then the assembly is heated to a higher temperature where final alloying takes place.

As can be seen from FIGURE 1, the bottom surface of the top plate 22 is in contact with the upper surface of the wafer 26 throughout the alloying process. Thus, the Walls of the aperture 25 serve both as a locating means for the pellet 27 and as a physical constraint thereon to define its area of contact with the surface of the wafer 26 throughout the alloying process.

It can be readily seen that this prior art method suffered from the disadvantages set forth above. The walls of the aperture 27 acting as a constraint on the pellet 27 tended to entrap oxygen at the points of constraint thereby tending to prevent uniform wetting of the wafer surface by the melted pellet. The large area of physical contact between the pellet 27 and the walls of the aperture 25 during alloying also encouraged sticking. With the area of contact between the pellet 27 and the wafer 26 defined by the constraining action, the depth of penetration of the melted pellet into the wafer was more difficult to control. The fact that separate heating cycles were used made it difficult to ensure that the two pellets would be in axial alignment with each other at their point of contact with the surfaces of the wafer.

Applicants, by eliminating these physical constraints during the alloying process, have overcome the disadvantages of the prior art. FIGURES 2, 3 and 4 typically illustrate the relationship between the wetting angle, the temperature and the depth of alloying, in the absence of physical constraints.

FIGURE 2 shows a typical example of the wetting angle 0 between an indium pellet 27 and a surface of a germanium wafer 26 after alloying has been completed to form a regrowth layer of p-type conductivity Zone 28. FIGURE 3 shows how the wetting angle falls continuously with increasing temperature. FIGURE 4 shows how greater control of the depth of penetration of the indium into the germanium can be accomplished in the unconstrained case than in the constrained case. For temperatures below the melting point of inditun in either case (nominally 155 C.), there is no flow of indium and hence no penetration takes place. At the high end of the usual range of alloying temperatures, the area of contact becomes so large in the unconstrained case that for a given volume of indium, very little increase in penetration takes place.

It is evident from FIGURE 3 that a lower wetting angle 6 'will always be the result of an increase in temperature and it follows that a larger area or" contact will occur at a higher temperature.

In the constrained case the finished alloy constituting the regrowth layer beneath the germanium surface takes the form of a circular prism having a radius R and a depth D Its alloyed volume would be defined by the constrained area and by the depth of alloying and therefore, would be:

AV 1rR AD since the radius R does not change due to the presence of constraints.

Similarly, in the unconstrained case, where the circular prism has a radius R and a depth D its alloyed volume would be:

and for the same increase in temperature AT, the change in volume would be:

since the radius R does change in the absence in the absence of constraints.

Since the same change in temperature for both the constrained and unconstrained cases would produce an equal change in alloyed volume regardless of the dimensions of the finished alloys, then The above mathematical analysis shows that a smaller variation of depth will occur in the unconstrained case whenever a variation in temperature occurs. This is illustrated graphically in FIGURE 4.

The actual depth of penetration of the indium into the regrowth layer 28 (FIGURE 2) is given by X=F(T)h for the constrained case :and by for the unconstrained case, where h is the height of-the alloyed pellet, D is the diameter of the sphere and 0 is the wetting angle in degrees (see the article Calculations of Alloying Depth of Indium in Germanium by T. Pensak, Transistor I, published by RCA Laboratories, Princeton, NJ.)

The two curves of FIGURE 4 were prepared from the above two equations using:

T I"(T) derived from the phase diagram of germanium and indium, also given graphically in the reference text.

In the unconstrained case, for example, with a 27 mil. diameter sphere of indium, a wetting angle of 75 will correspond to a 475 C. alloying temperature. The area of contact A will be about ZVsD where D is the diameter of the sphere. The depth of penetration can be controlled by fixing the wetting angle and temperature and varying the size of the indium sphere.

Referring to FIGURES 5, 6 and 7 for a detailed description of one embodiment of applicants invention, a jig 30 comprises a bottom plate 31 and a top plate 32, each preferably made of suitably oxide-coated grade 310 stainless steel. The bottom plate 31 has a top and bottom surface and a first cavity 33 of cylindrical cross-section cut in its top surface to a predetermined depth to define a bottom for the cavity 33. The plate 31 has a second cavity 34 of predetermined conical cross-section cut in the bottom of the first cavity 33 with the sides of the cavity 34 tapering toward the bottom surface of the plate 31. A

ventilation hole 35 is coaxial with the cavity 34 and extends to the bottom of the plate 31. The top plate has a top and bottom surface and a cylindrical-shaped aperture 36 of predetermined cross-section extending therethrough.

The cavity 34 serves as a first locating means for initially locating a pellet of electrode-forming material, shown as an indium sphere 37, at one surface of a semiconductive body, shown as a small wafer of N-type germanium 38 having its opposite surfaces cut in the 1.1.1 plane. The cavity 33 serves as a means for supporting the wafer 38. The aperture 36 which has a cross-section just large enough to pass a second pellet of electrode-forming material therethrough, serves as a second locating means for initially locating the second pellet, shown as an indium sphere 39, at the opposite surface of the wafer 38. The top plate 32 is referenced with respect to the bottom plate 31 such that the aperture 36 and the cavity 34 are in axial alignment with each other. To ensure that the sphere 39 will lose physical contact with the walls of the aperture 36 during the alloying process, the depth of the cavity 33 and the cross-section of the cavity 34 must be such that before alloying, the sphere 39 will rest on the upper surface of the wafer 38 approximately one radius below the bottom surface of the top plate 32. If much more than one radius is allowed, there will be too much lateral movement of the sphere 39 making it difiicult to provide axial alignment with the sphere 37. If much less than one radius is allowed, the walls of the aperture 36 will act as a constraint on the sphere 39 during alloying.

The embodiment shown in FIGURES -7 contemplates a method for manufacturing a P-N-P germanium transistor in one alloying cycle and includes means for mounting the transistor and includes means for mounting the transistor and means for attaching leads to the base region of the transistor. For this purpose, the cavity 33 has adjacent its walls a pair of parallel grooves 40 and 41 cut in its upper surface one on each side of the cavity 34.

Referring specifically to FIGURE 5 for the loading steps prior to alloying the first indium sphere 37 is placed in the cavity 34, nickel wires 42 and 43 are laid in the grooves 40 and 41 respectively and a gold ring 44 is placed in the cavity 33 around the sphere 37 in contact with the wires 42 and 43. The wafer 38 is then placed in the cavity 33. The depth of the cavity 33 and the cross-section of the cavity 34 must be such that the bottom surface of the wafer 38 will rest at one edge on the gold ring 44 and at approximately at its center on the sphere 37. This slight tilting of the wafer 38 (shown greatly exaggerated in FIGURE 5) is to ensure that its weight will be borne by the sphere 37. The top plate 32 is placed on the bottom plate 31 and precisely referenced by dowels. The sphere 39 is dropped through the aperture 36 to rest on the upper surface of the wafer 38. The tilting of the wafer 38 is so slight that the axes of the spheres 37 and 39 normal with the surfaces of the wafer 38 will be substantially in alignment.

In a typical heating cycle the assembled jig 30 is placed in an oven and heated to a temperature of 475 C. in 20 minutes in an atmosphere, say, of 25% forming gas (25% H -75% N retained at this temperature for minutes, and then cooled to room tempearture in minutes.

During the heating cycle the spheres 37 and 39 reach their melting point at about 155 C. The wafer 38 descends by its own weight into contact with the gold ring 44. The sphere 37 is drawn upwards by surface tension forces acting against gravity between the bottom surface of the wafer 38 and the sphere 37. The sphere 37 thus becomes free of the cavity 34 and natural physical forces take effect during alloying to give the alloyed pellet its final shape. The sphere 39 drops free from the aperture 36 due to gravity aided by surface tension forces acting between the top surface of the wafer 38 and the sphere 39. Again, natural physical forces take effect during alloying without physical constraints. In addition the gold ring 44 is thermally bonded to the wafer 38 and the wires 42 and 43 fuse with the gold ring 44. The assembled jig after alloying is completed is shown in FIGURE 6. Regrowth layers of P-type conductivity zones 45 and 46 are formed. The sphere 37 and the zone 45 form the collector electrode of the transistor. Similarly, the sphere 39 and the zone 46 form the emitter electrode of the transistor. The original N-type conductivity germanium is the base electrode.

FIGURE 7 shows the gold ring 44 bonded to, and the sphere 37 alloyed to the bottom surface of the wafer 38, and the wires 42 and 43 fused to the gold ring 44. The jig is unloaded by removing the top plate 32 and lifting the transistor out by the wires 42 and 43. Suitable leads may be subsequently bonded to the indium spheres 37 and 39, to complete the electrical connections to the transistor.

FIGURES 8-12 illustrate another embodiment of applicants invention incorporating the same basic principles. FIGURE 8 is a perspective view of a bottom plate 51 of a jig 50. FIGURES 9 and 10 show the assembled jig with a top plate 52 added before alloying and FIGURES 11 and 12 show the assembled jig after alloying. Again the plates 51 and 52 are preferably made of suitably oxide-coated grade 310 stainless steel.

The bottom plate 51 (FIGURE 8) has a first groove 53 of predetermined width cut in its upper surface to a first predetermined depth and a second groove 54 of predetermined width cut in its upper surface to a second predetermined depth below the first predetermined depth to intersect the groove 53. The grooves 53 and 54 are shown intersecting at right angles but this is not absolutely necessary. The groove 54 has a pair of additional grooves 55 and 56 of predetermined substantially equal width cut in its upper surface. The point of intersection of the grooves 55 and 56 are substantially equidistant to each wall of the groove 53 and substantially equidistant to each wall of the groove 54. In practice, the groove 55 is cut centrally along the length of the groove 53 below the second predetermined depth and the groove 56 is cut centrally along the length of the groove 54 below the second predetermined depth.

Thus, the grooves 55 and 56 intersect at right angles. These grooves also serve to increase the circulation of the reducing gas during alloying. The top plate 52 (FIGURES 911) as in the first embodiment has a cylindrical-shaped aperture 57 of predetermined cross-section. The top plate 52 is arranged to rest at its bottom surface upon the top surface of the bottom plate 51 such that the aperture 57 is in axial alignment with the point of intersection of the grooves 55 and 56.

This point of intersection serves as a first locating means for initially locating a pellet of electrode-forming material shown as an indium sphere 58 at one surface of a semiconductive body. A nickel bracket 60 having a downwardly extending seat 61 with an aperture 62 therein is adapted to receive a wafer of semiconductive material, shown as a small wafer of N-type germanium 59 having its opposite surfaces cut in the 1.1.1 plane. The groove 53 is arranged to receive the bracket 60 which serves as a means for supporting the wafer 59. The width of the groove 53 is such as to inhibit transverse movement of the bracket 60 therein.

The bracket 60 can be made of other materials than nickel, the important requirement being that the material chosen should have good electrical conductivity and good thermal conductivity properties.

The width of the groove 54 is such that the seat 61 can extend therein above the point of intersection of the grooves 55 and 56.

The aperture 57 which has a cross-section just large enough to pass a second pellet of electrode-forming material therethrough, serves as a second locating means for initially locating the second pellet, shown as an indium sphere 63, at the opposite surface of the wafer 59.

The depths of the grooves 53 and 54 and the widths of the grooves 55 and 56 must be such that before alloying the bottom surface of the wafer 59 will reset at one edge on the seat 61 and at approximately its center on the sphere 58 and the sphere 63 when dropped in the aperture 57 will rest on the top surface of the Wafer 59 approximately one radius below the bottom surface of the plate 52.

Referring specifically to FIGURES 9 and for the loading steps prior to alloying, the first sphere 58 is placed at the point of intersection of the grooves 55 and 56 where it receives four points of support. The use of intersecting grooves further reduces the tendency for sticking and provides good freedom for circulation of the reducing gas which benefits good wetting. The bracket 60 is placed along the groove 53 with the aperture 62 of the seat 61 in axial alignment with the point of intersection of the grooves 55 and 56. Transverse movement of the bracket 60 is prevented by the walls of the groove 53. The wafer 59 is then placed in the seat 61 with the bottom surface of the wafer 59 resting at one edge on the seat and approximately at its center on the sphere 58. The top plate 52 is placed on the bottom plate 51 and precisely referenced by dowels. The sphere 63 is dropped through the aperture 57 to rest on the upper surface of the wafer 59.

The assembly is now ready for alloying and the heating cycle may, for example, be the same typical one described with reference to the embodiment of FIGURES 5-7. In a similar manner, when the spheres 58 and 63 reach their melting point, the wafer 59 descends by its own weight into contact with the bracket 60 and is thermally bonded thereto. The sphere 58 is drawn upwards by surface tension forces acting against gravity between the bottom surface of the wafer 59 and the sphere 58. The sphere 63 drops free from the aperture 57 due to gravity aided by surface tension forces acting between the top surface of the wafer 59 and the sphere 63. With both spheres free of the jig 50, natural physical forces take effect during alloying without physical constraints.

The assembled jig after alloying is completed is shown in FIGURE 11. FIGURE 12 shows the sphere 58 and the seat 61 of the bracket 60 bonded to the bottom surface of the wafer 59. The jig 50 is unloaded by removing the top plate 52 and overturning the bottom plate 51. To give some idea of the size of the jig and the number of transistors that can be made in one alloying cycle, applicants have succeeded in making a 4 inch x 1 inch jig having 80 individual transistor alloying stations.

In both embodiments of the invention the sphere 39 (FIGURES 5 and 6) or the sphere 63 (FIGURES 9-11) fall free of their respective apertures and begin to alloy With the top surface of the wafer about at the melting point of the indium. However, it is within the scope of the invention to have these spheres fall free and begin alloying at some temperature between the melting point of the indium and a predetermined maximum alloying temperature. For this purpose the top plate could have inwardly directed flanges at the bottom of the aperture to retain the sphere until some higher temperature is reached. The more inwardly directed the flanges are, the higher will be the temperature at which the melted spheres fall free of the aperture.

To minimize sticking problems applicants have deposited an oxide on the surface of the stainless steel by chemical oxidization in nitric acid followed by heating in air. In practice, it has been found necessary to replenish the oxide coating every 20 to 30 alloying operations. Applicants have found that only the grades of stainless steel that have the highest percentage of chrome, such as grade 310, are satisfactory for use as jigs.

Thus, by eliminating physical constraints, applicants have provided a new method for alloying an electrode with a semiconductive body and a new jig for use in carrying out the method whereby improved wetting, less sticking and better control of depth are accomplished.

In addition, collector and emitter electrodes of a transistor can be alloyed in precise axial alignment with each other by applicants novel one cycle alloying process.

What is claimed is:

1. A method for alloying an electrode with a semiconductive body comprising the steps of initially supporting and locating a pellet of electrode-forming material at one surface of a semiconductive body such that the pellet will lose physical contact with the support therefor and begin alloying with the surface of the semiconductive body at a temperature between the melting point of the pellet and a predetermined maximum alloying temperature, and then raising the temperature of the pellet and semiconductive body in an atmosphere conducive to alloying to the predetermined maximum alloying temperature.

2. A method for alloying a pair of electrodes with a semiconductive body comprising the steps of initially supporting and locating a pair of pellets of electrode-form ing material at opposite surfaces of a semiconductive body such that the pellets will lose physical contact with the Support therefor and begin alloying with the opposite surfaces of the semiconductive body at a temperature between the melting point of the pellets and a predetermined maximum alloying temperature, the pellets being initially located substantially opposite each other, and then raising the temperature of the pellets and semiconductive body in an atmosphere conducive to alloying to the predetermined maximum alloying temperature.

3. A method as defined in claim 2 wherein the pellets are substantially spherical, the axis of one located pellet normal to one surface of the semiconductive body being substantially in alignment with the axis of the other located pellet normal to the opposite surface of the semiconductive body.

4. A method as defined in claim 3 wherein the pellets are indium spheres and the semiconductive body is a crystal of N-type germanium having its opposite surfaces cut in the 1.1.1 plane.

5. A method as defined in claim 4 wherein one pellet is initially located to rest on the one surface of the semiconductive body and the other pellet is initially located to rest on the opposite surface of the semiconductive body and the pellets will lose physical contact with the locating jig about at the melting point of the pellets.

No references cited.

DAVID L. RECK, Primary Examiner.

R. O. DEAN, Assistant Examiner. 

1. A METHOD FOR ALLOYING AN ELECTRODE WITH A SEMICONDUCTIVE BODY COMPRISING THE STEPS OF INITIALLY SUPPORTING AND LOCATING A PELLET OF ELECTRODE-FORMING MATERIAL AT ONE SURFACE OF A SEMICONDUCTIVE BODY SUCH THAT THE PELLET WILL LOSE PHYSICAL CONTACT WITH THE SUPPORT THEREFOR AND BEGIN ALLOYING WITH THE SURFACE OF THE SEMICONDUCTIVE BODY AT A TEMPERATURE BETWEEN THE MELTING POINT OF THE PELLET AND A PREDETERMINED MAXIMUM ALLOYING TEMPERATURE, AND THEN RAISING THE TEMPERATURE OF THE PELLET AND SEMICONDUCTIVE BODY IN AN ATMOSPHERE CONDUCIVE TO ALLOYING TO THE PRE DETERMINED MAXIMUM ALLOYING TEMPERATURE. 